A so-called terne-coated steel sheet coated with a lead-tin alloy formed by adding 3 to 25% of tin to lead has been heretofore used for, for instance, automobile fuel vessels (gasoline tanks). This steel sheet has a good corrosion resistance and processability and is advantageous from an economical viewpoint.
Due to recent problems with the supply conditions of petroleum (increase in the cost of and decrease in the amount produced petroleum), and in line with present efforts to find an alternative to gasoline as a fuel, attempts have been made to use an alcohol fuel such as methyl alcohol or ethyl alcohol, or a mixed fuel (so-called gasohol) formed by incorporating an alcohol such as methyl alcohol, ethyl alcohol or isopropyl alcohol into gasoline, instead of gasoline.
The lead-tin alloy-coated steel sheet heretofore used for an automobile fuel vessel does not have sufficient corrosion resistance when used for a vessel for containing such as alcohol fuel or alcohol-added gasoline (gasohol).
More specifically, the lead-tin alloy-coated steel has a covering layer formed of a lead-tin eutectic alloy composed mainly of lead, and the corrosion resistance of the lead-tin alloy-coated steel sheet to alcohol-containing fuels is drastically degraded, mainly for the following reasons.
(1) The corrosion resistance of the lead metal to methyl alcohol, ethyl alcohol, and the like, is very low and lead metal is severely corroded by such an alcohol. Accordingly, the lead metal portion in the lead-tin alloy covering layer is easily corroded. PA1 (2) Usually, an alcohol contains water. When an alcohol is incorporated in gasoline, a phase containing water in a large amount separates from the gasoline-alcohol mixed phase. Therefore, if pinholes are formed in the lead-tin alloy covering layer, corrosion of the lead-tin alloy covering layer by the water-containing phase is promoted. PA1 (1) By laminating the first, second, and third layers, the number of pinholes in the covering is kept very low. PA1 (2) The metal or alloy constituting the third covering layer forming the surface of the coated steel sheet has a high sacrificing anticorrosive effect against water of chlorine ions (Cl.sup.-) contained in fuel or air, which cause pitting in the coated steel sheet. Accordingly, even if pinholes are present in the third covering layer, this sacrificing anticorrosive effect prevents corrosion of the first and second covering layers. PA1 (3) It is important that at least first and second covering layers should be present between the steel sheet in the substrate and the third covering layer. Namely, the metal used for formation of the third covering layer is potentially nobler than steel, and therefore, if a covering layer having the same composition as that of the third covering layer is directly formed on the steel sheet, corrosion of the steel sheet in the pinhole portion is promptly advanced and there is a great risk of red rust or pitting. In order to solve this problem, it is necessary that the thickness of the covering layer should be increased. However, an increase of the thickness of this covering layer will result in a reduction of the forming processability of the obtained coated steel sheet, and the product becomes disadvantageous from the economical viewpoint. PA1 (4) Since the first covering layer (lead-tin alloy layer) formed on the substrate is soft and has an excellent lubricating property, the obtained coated steel sheet has a good forming processability and the formation of cracks extending to the surface is controlled. This effect of controlling the formation of cracks provides a great improvement in the corrosion resistance of the coated steel sheet. PA1 (1) No covering layer is formed and therefore, the surface of the steel sheet substrate is exposed. PA1 (2) Only the first covering layer is formed. PA1 (3) The first and second covering layers are formed. PA1 (4) A covering layer composed of a zinc-nickel (8 to 20% by weight) alloy, a zinc-cobalt (8 to 20% by weight) alloy, a zinc-nickel or cobalt (8 to 20% by weight) alloy, or a zinc-iron (8 to 20% by weight) alloy is formed.
Accordingly, a material in which a much smaller number of pinholes are formed in the covering layer, compared with the number of pinholes formed in the conventional material, and having an excellent resistance to corrosion by an alcohol or alcohol oxide, is strongly desired as a material for forming a vessel for an alcohol-containing fuel, for example, an automobile fuel tank.
The need to improve the corrosion resistance in a material of a fuel vessel is increasing, not only for the inner surface of the fuel vessel but also for the outer surface of the fuel vessel.
More specifically, it is necessary to develop a highly anticorrosive material for forming a covering layer in which the number of pinholes is reduced, and which has a high resistance to corrosion caused by salt scattered over the surface of roads, etc., to prevent icing thereof in winter.
As a material satisfying this requirement, we previously produced a steel sheet having a surface covering layer composed of tin, cobalt, nickel, or an alloy thereof, which as an excellent resistance to corrosion by an alcohol or alcohol-containing fuel, this surface covering layer being formed on a covering layer composed of a lead-tin alloy, as shown in Japanese Unexamined Patent Publication (Kokai) No. 59-104496, and relatively good results were obtained according to this proposal.
The above steel sheet is characterized in that, in order to solve the problem of a low corrosion resistance to alcohol-containing fuel or alcohol fuel in a lead-tin alloy-coated steel sheet heretofore used for an automobile fuel vessel, a covering layer composed of tin, nickel, cobalt, or an alloy thereof, which has an excellent resistance to corrosion by alcohol fuel or alcohol-containing fuel, is formed as the surface layer covering the lead-tin alloy-coating layer. Furthermore, other characteristics (for example, forming-processability, solderability, and weldability) of the lead-tin alloy-coated steel sheet required for the fuel vessel material are retained in this fuel vessel material, and a fuel vessel material practically applicable to an alcohol fuel or alcohol-containing fuel vessel can be provided.
However, from the results of experiments, it has been found that when this steel sheet is exposed to an alcohol-containing fuel, an alcohol fuel, or water containing a chlorine ion (Cl.sup.-), a satisfactory and stable corrosion resistance cannot be always obtained.
More specifically, in the above steel sheet, since pinholes in the covering layer of the above-mentioned metal or alloy of the lead-tin alloy-coated steel sheet are enlarged according to the processed shape of the fuel vessel, red rust often appears on the steel sheet. The water content in alcohol-containing fuel is high (at least about 0.75% based on the alcohol contained in the fuel), and when the phase containing water in a large amount is separated from the other phase, red rust spots often appear in the portion of the steel sheet in contact with the water-containing phase. Thus, it has been confirmed that the above coated steel sheet still does not have sufficient resistance to corrosion by the above-mentioned fuels.
Moreover, in a corrosion test of the outer surface of the fuel vessel with an aqueous solution containing a chlorine ion (Cl.sup.-), it was found that many spots of red rust appear on the outer surface of the vessel which has been subjected to a severe forming process.
Accordingly, where pinholes extending to the substrate of the steel sheet are present, pitting corrosion is caused in the substrate of the steel sheet contained in the fuel or the Cl.sup.- ion or water contained in the surrounding atmosphere, and the above-mentioned coated steel sheet is defective in that the life of the steel sheet vessel against such corrosion is not practically sufficient.